Bc. Ivo Puffer of the Czech
Republic has prompted me to look more carefully at how the
Moon’s
behaviour changes during its 18.6-year cycle.

The Sun moves 23.4 degrees
north of the equator in midsummer, and 23.4 degrees south of it in
mid-winter; its path, the ecliptic, is shown as the
red line
on the graphs below. 23.4 degrees is “the obliquity of the
ecliptic”, often written as ε.

The Moon’s path takes it
first north of the the ecliptic, then south, by up to 5.1 degrees;
this is the “inclination of the Moon’s
orbit”, often written i.
The Moon’s path crosses the ecliptic at two places, the ascending
node, where it crosses the ecliptic from south to north, and
the
descending node, where it goes the other way.
And the nodes
move slowly westwards along the ecliptic.

A “major standstill”
occurs when the Moon’s ascending node is at 0h Right
Ascension –
the point where the Sun is at the March equinox. As the graph below
shows, at a major standstill the Moon (the blue line) reaches further
north, and further south than the
Sun, by an additional
5.1 degrees. This occurred, for example, in May 1988.

As
the Moon’s nodes drift westward along the ecliptic, the
crossing-point moves. Here it is about three years later (e.g. 1991),
at 20h Right Ascension (roughly where the Sun is in the middle of
January). Although the Moon still reaches farther north and south
than the Sun, it’s not getting quite so far.

After
about four and a half years (for example, in late 1992) the
Moon’s
ascending node is at 18h Right Ascension: it crosses the ecliptic at
the point the Sun reaches at the December solstice. The
Moon’s
range is now almost the same as the Sun’s, 23.4 degrees north
and
south. (It’s not quite the same, because
the Moon is little
above the ecliptic just before its peak. The range is exactly the
same a few months later, when the node is at about 17h35m.)

After
another four and a half years, we reach a “minor
standstill”: the
Moon crosses the ecliptic from south to north at Right Ascension 12h
– the point where the Sun is at the September equinox. Now
the Moon
reaches only (23.4 - 5.1) = 18.3 degrees north or south of the
equator. This occurred in February 1997.

Here’s the position about
three years later (e.g. 2000): the node has reached 8h Right
Ascension, roughly where the Sun is in late July.

When
it gets round to 6h, the position of the Sun at the June solstice (in
late 2001, for example), the ranges are almost equal again. (They are
exactly equal a few months earlier, when the node is at Right
Ascension 6h25m).

Finally
the ascending node gets back to 0h and the cycle is complete, 18.6
years on. And we have another major standstill in June 2006. The next
one will be in April 2025.

This discussion ignores many
of the complex feature of the Moon's behaviour. Note, for example,
that both the obliquity of the ecliptic and the Moon’s
inclination
change very slowly, so the values given here are only correct for the
present time. Two thousand years ago, when many great megalithic
monuments were being constructed, the values were somewhat different,
so the points on the horizon where the Sun and Moon rose and set were
different from today.